DESCRIPTION (Adapted from applicant's description): The specific aims of the present investigation are to determine: 1) reliability of three physical activity assessment instruments (heart rate (HR) monitoring. Caltrac accelerometry, and physical activity recall (PAR) in women during pregnancy and the postpartum periods; 2) convergent validity of three physical activity assessment instruments in women during pregnancy and postpartum periods; 3) construct validity of three physical activity assessment instruments in women during pregnancy and the postpartum period; and 4) HR/oxygen consumption (VO2; i.e., energy expenditure) calibration curves for each subject during pregnancy and postpartum. The questions of primary interest are: 1) Do estimates provided by the three instruments provide consistent rank-ordering of the energy expenditures of pregnant women? 2) Are standard metabolic efficiency conversion tables appropriate for pregnant women? and 3) Does the efficiency of energy utilization change during pregnancy, and is this change modified by habitual physical activity?